This invention relates to the incorporation of a fibrillated acrylic polymer pulp fiber into non-asbestos type friction materials for the purpose of improving the structural integrity of preforms. The preforms serve as intermediate products in the manufacture of friction elements.
It has become desirable to find a replacement for asbestos containing friction materials because of the health and safety hazards attributed to asbestos. Numerous approaches to the replacement of asbestos have led to a substantial body of technology that has resulted in at least three major categories of non-asbestos type formulations. The are: (1) semi-metallic materials, (2) organic non-asbestos materials, and (3) cold molding hydrocarbon materials. Such categories are generally illustrated by U.S. Pat. Nos. 3,856,120; 4,137,214 and 4,125,496, respectively. Other typical non-asbestos formulations are included in U.S. Pat. Nos. 4,278,584; 4,226,758; 4,226,759; and 4,219,452, all of which patents are hereby incorporated herein by reference.
The elimination of asbestos from friction material formulations has, however, created a substantial manufacturing problem. The problem is that the non-asbestos containing preforms, normally pressure formed at ambient temperatures prior to hot pressing and heat curing, generally do not possess acceptable structural integrity so as to enable them to withstand subsequent handling and storage without breakage. On the other hand, asbestos-containing compositions possess the requisite structural integrity to withstand such handling.
Attempts to substitute natural and synthetic fibers for asbestos fibers in the manufacture of friction materials are also exemplified by the following publications: U.S. Pat. No. 4,145,223 wherein the incorporation of glass fibers, steel fibers, organic synthetic fibers such as of phenolic resins and ceramic fibers is disclosed; U.K. Published Application No. 2027724 A wherein preoxidized acrylic fibers are taught; U.S. Pat. No. 4,197,223 and U.K. Pat. No. 1604827 wherein mixtures of inorganic and organic fibers such as glass fibers, mineral wools, alumino-silicate fibers, wood pulp, jute, sisal or cotton linters fibers are taught; U.S. Pat. Nos. 4,374,211 and 4,384,640 which teach aramid polymers; U.S. Pat. Nos. 4,418,115; 4,508,855; 4,539,240; 4,656,203; G.B. Published Application No. 2129006A; Japanese Published Application 87/106133; Japanese Published Application 87/89784; and Japanese Published Application 87/149908, all of which teach various acrylic polymer fiber incorporation, and all of which are also incorporated herein by reference.
All of these references fail, however, to recognize the critical correlation between fiber length and Canadian Standard Freeness (CSF) of the acrylic fiber which forms the crux of the inventive concept set forth herein. This invention solves the structural integrity problem encountered in non-asbestos preforms with the expedient of incorporating an effective amount of a fibrillated acrylic fiber having a unique fiber length/CSF correlation into the friction material to ensure the attainment of sufficient structural integrity to withstand the subsequent handling and/or storage of the preform prior to further processing.